Solar water heating systems use heat exchangers to transfer solar energy absorbed in solar collectors to potable (drinkable) water.
Heat exchangers can be made of steel, copper, bronze, stainless steel, aluminum, or cast iron. Solar heating systems usually use copper, because it is a good thermal conductor and has greater resistance to corrosion. Stainless steel is also common in “compact” heat exchangers.
Types of Heat Exchangers
Solar water heating systems use three types of heat exchangers:
A liquid-to-liquid heat exchanger uses a heat-transfer fluid (often a mixture of propylene glycol and water) that circulates through the solar collector, absorbs heat, and then flows through a heat exchanger to transfer its heat to potable water in a storage tank. Heat-transfer fluids, such as propylene glycol antifreeze, protect the solar collector from freezing in cold weather. Liquid-to-liquid heat exchangers have either one or two barriers (single wall or double wall) between the heat-transfer fluid and the domestic water supply. A “double-wall” heat exchanger will drain any leak of heat transfer fluid from a gap before it could get into the potable water.
A single-wall heat exchanger is a pipe or tube surrounded by a fluid. Either the fluid passing through the tubing or the fluid surrounding the tubing can be the heat-transfer fluid, while the other fluid is the potable water. An easy way to construct such a heat-exchanger is to put a small pipe into a larger pipe and circulate the fluid to be heated in the outer pipe.
Double-wall heat exchangers have two walls between the two fluids. Two walls with drainage between the two and leak detection is required when the heat-transfer fluid is toxic, and are often used even with non-toxic heat transfer fluids such as propylene glycol (antifreeze). Double walls are required as a safety measure in case of leaks, helping ensure that the antifreeze does not mix with the potable water supply. An example of a double-wall, liquid-to-liquid heat exchanger is the "wrap-around heat exchanger," in which a tube is wrapped around and bonded to the outside of a hot water tank. The tube must be adequately insulated to reduce heat losses.
While double-wall heat exchangers increase safety, they are less efficient because heat must transfer through two surfaces rather than one. To transfer the same amount of heat, a double-wall heat exchanger must be larger than a single-wall exchanger.
- Air-to-liquid or liquid to-air
Solar heating systems with air-heating solar collectors usually do not need a heat exchanger between the solar collector and the air distribution system. Those systems with air heater collectors that heat water use air-to-liquid heat exchangers, which are similar to liquid-to-air heat exchangers. These are similar in appearance to the radiator on the front of a large truck engine.
Heat Exchanger Designs
The heat exchanger is a coil of tubing in the storage tank. It can be a single tube (single-wall heat exchanger) or the thickness of two tubes (double-wall heat exchanger)), depending on the heat transfer fluid. . A less efficient alternative is to place the coil on the outside of the collector tank with a cover of insulation.
The heat exchanger is separate from (external to) the storage tank. It has two separate fluid loops inside a case or shell. The fluids flow in opposite directions to each other through the heat exchanger, maximizing heat transfer. The potable water to be heated circulates through a shell surrounding the tubes and the heat transfer fluid from the solar collectors circulates through the inner tubes. The tubes and shell should be made of the same material. When the collector or heat-transfer fluid is toxic, double-wall tubes are used, and a gap is placed between the outer and inner walls of the tubes.
In this very efficient design, the tubes of water and the heat-transfer fluid are in direct thermal contact with each other. A tube-in-tube heat exchanger is created by putting a small pipe into a larger pipe, and the assembly may be coiled to occupy less space. The water and the heat-transfer fluid flow in opposite directions to each other. This type of heat exchanger has two loops similar to those described in the shell-and-tube heat exchanger.
- “Compact” heat exchanger
In so-called “compact” heat exchangers a very large surface area is created by plates stamped out of stainless steel. Because of the strength and corrosion-resistance of stainless stell these plates can be very thin and close together.
A heat exchanger must be sized correctly to be effective. There are many factors to consider for proper sizing, including the following:
- Type of heat exchanger
- Characteristics of the heat-transfer fluid (specific heat, viscosity, and density)
- Flow rate
- Inlet and outlet temperatures for each fluid.
Usually, manufacturers will supply heat transfer ratings for their heat exchangers (in Btu/hour) for various fluid temperatures and flow rates. Also, the size of a heat exchanger's surface area affects its speed and efficiency: a large surface area transfers heat faster and more efficiently. There are two methods to size heat exchangers:
- the log-mean temperature difference method divides required heat transfer rate by a log-mean incoming and exiting temperature differences and by heat transfer coefficient to determine the required surface area and the;
- “effectiveness” method where the required size is the required heat transfer rate divided by an “effectiveness” and by the maximum temperature difference (hot solar minus cold water). Effectiveness depends on the heat transfer coefficient and flow rates and is typically on the order of 60%.
A designer might use calculators provided on supplier’s websites to do these calculations.
For the best performance, always follow the manufacturer's installation recommendations for the heat exchanger. Be sure to choose a heat-transfer fluid that is compatible with the type of heat exchanger you will be using. If you want to build your own heat exchanger, be aware that using different metals in heat exchanger construction may cause corrosion. Also, because dissimilar metals have different thermal expansion and contraction characteristics, leaks or cracks may develop. Either of these conditions may reduce the life span of your heat exchanger.
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